Conventionally, in a wireless communication system, to prevent the occurrence of data packet loss during handover of a mobile device, a system that performs a packet transfer between base stations (hereinafter, “packet-transfer handover system”) has been proposed. The packet-transfer handover system is explained below.
In a wireless access network of a wireless communication system such as the IMT (International Mobile Telecommunication) 2000 and the CDMA (Code Division Multiple Access) 2000, a mobile device performs communications with a base station while moving between communication areas called “cell” generated by plural base stations. When the mobile device reaches near a cell boundary and when the mobile device becomes difficult to continue communication with a base station because of a degraded quality of wireless communication, the mobile device is handed over to another base station having a better quality of wireless communication.
In the packet-transfer handover system, a handover-source base station to which a mobile device is connected before performing handover transfers an untransmitted data packet destined to the mobile device and remaining in own base station, to a handover-destination base station before performing the handover of the mobile device. The handover-destination base station transmits the transferred data packet. By transferring the data packet destined to the mobile device between the base stations in this way, the occurrence of a loss of the data packet destined to the mobile device during handover is prevented.
However, according to the packet-transfer handover system, a transfer delay corresponding to a time required for a handover control process and a transfer between base stations occurs in an inter-base-station transfer packet. Therefore, at the time of handling real-time data such as streaming, there is a risk of losing a real-time characteristic of communications. As a method of maintaining a real-time characteristic of communications during handover, Patent Document 1 mentioned below discloses a method of controlling a transmission timing of a packet from a handover-source base station, by adjusting a process delay due to handover to an inter-base-station transfer packet at the handover. Further, Patent Document 2 mentioned below discloses a method of preventing an increased delay of an inter-base-station transfer packet, by transmitting a packet to be transferred between base stations with priority over a non-transferred packet.
Meanwhile, in a base station of such a wireless communication system, priority control is performed between multiple users corresponding to quality of service (QoS) at the time of transmitting data to a wireless transmission path. As one of priority control methods, there is a priority control of determining a priority of wireless transmission (hereinafter, “delay priority control”) based on an upper limit (a delay tolerance) to a device retention time of a data packet. The device retention time is a time during which a data packet remains in a buffer within the device. It is assumed that the device retention time indicates a time from a time (a receiving time) when the device receives a data packet until a time when the device calculates a wireless transmission priority. In the delay priority control, when the device retention time is nearer the permissible delay, a data packet is transferred with priority by increasing a value of the wireless transmission priority, and the device retention time is controlled not to exceed the permissible delay.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-304325    Patent Document 2: International Publication No. WO04/030396 Pamphlet